Compositions and Methods for Hindering Asphaltene Deposition

ABSTRACT

Methods and compositions for hindering asphaltene deposition. One method comprising: identifying an interval of the subterranean formation to be treated with a relative permeability modifier to hinder subsequent asphaltene deposition; introducing the relative permeability modifier into the subterranean formation; and allowing the relative permeability modifier to contact the interval, thereby attaching to surfaces within the subterranean formation and hindering subsequent asphaltene deposition.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/097,057, filed Sep. 15, 2008, entitled “Compositions and Methods for Hindering Asphaltene Deposition,” the entire disclosure of which is incorporated herein by reference.

BACKGROUND

The present invention relates to subterranean treatments and, more particularly, in one or more embodiments, to introducing a relative permeability modifier into a subterranean interval, optionally in conjunction with an asphaltene solvent system, to hinder asphaltene deposition.

The formation of asphaltene deposits may be a problem in crude oil production. Asphaltene precipitation and deposition may cause problems such as production loss due to the asphaltenes plugging the tubing, perforations, and portions of the subterranean formation. As used in this disclosure, the term “asphaltene” refers to organic material that may be found in hydrocarbon deposits (e.g., petroleum, crude oil) and that generally comprise aromatic and napthenic ring compounds and/or waxes. Asphaltenes may be found in crude oil in the form of colloidal, suspended, solid particles. Asphaltenes may be characterized by their insolubility in light paraffin hydrocarbon solvents. These compounds typically may have high molecular weights and may be polar materials because atoms of sulfur, nitrogen, oxygen, and complex metals may be present in their structures.

Asphaltene deposition may occur when the crude oil loses its capability to disperse and stabilize the asphaltene particles. The asphaltene stability may depend on factors such as the composition of the crude oil, temperature, pressure, and the nature of the reservoir rock surface (e.g. rock wettability). Under static reservoir conditions, asphaltenes may be held in a stable suspension by resins, a family of polar molecules. Changes in fluid temperature and pressure that can be associated with oil production from the reservoir may cause the asphaltenes to flocculate and precipitate out of suspension and adsorb to the rock or pipe surfaces. Additionally, the asphaltenes may flocculate because of electrical charges created by the motion of the flowing hydrocarbons. Asphaltene deposition may occur at anytime in the production life cycle.

Regardless of the mechanism causing the asphaltene to deposit, the result may be a plugging effect that inhibits or reduces oil production. Traditional methods of removing asphaltene deposits may involve heat, mechanical removal, dispersants, and/or solvents. The most common means of transmitting heat downhole is hot oiling. While hot oiling has been a popular method of paraffin removal, the process can cause significant formation damage particularly when there are asphaltenes present in the subterranean formation. Mechanical removal techniques include scarpers, cutters, and coil-tubing deployed jetting tools. Mechanical removal has the limitation of not being able to remove formation damage outside of the perforations. Dispersant systems do not dissolve asphaltenes; rather they dispense the particles so that they can be circulated from the well bore.

The solvents most frequently used for asphaltene cleanup are toluene and xylene. Clean up with pure toluene may remove the majority of the asphaltenes, but the surface on which the asphaltenes are adsorbed may still be covered with a layer of asphaltenes and/or remain oil wet. Xylene or xylene mixtures may have limited effectiveness in addition to undesirable health, safety, and environmental characteristics. Other problems associated with asphaltene solvent systems are that as the solvent removes the asphaltenes, losses can occur into the rock matrix. As the losses begin, several problems may occur which include, losses can be easily focused in thief zones, making complete coverage of solvents difficult, losses may occur too quickly and the solvent systems may not have a chance to completely dissolve well bore asphaltenes, and dissolved asphaltenes may be carried further into the matrix. Furthermore, even after effective asphaltene removal treatment, asphaltene deposition may continue to occur necessitating future remediation.

SUMMARY

The present invention relates to subterranean treatments and, more particularly, in one or more embodiments, to introducing a relative permeability modifier into a subterranean interval, optionally in conjunction with an asphaltene solvent system, to hinder asphaltene deposition.

In an embodiment, the present invention provides a method of hindering asphaltene deposition comprising: identifying an interval of the subterranean formation to be treated with a relative permeability modifier to hinder subsequent asphaltene deposition; introducing the relative permeability modifier into the subterranean formation; and allowing the relative permeability modifier to contact the interval, thereby attaching to surfaces within the subterranean formation and hindering subsequent asphaltene deposition.

In another embodiment, the present invention provides a method of removing asphaltenes and hindering subsequent asphaltene deposition in a subterranean formation comprising: introducing a fluid comprising a relative permeability modifier and an asphaltene solvent system into the subterranean formation; and allowing the fluid to contact a portion of the subterranean formation, wherein the asphaltene solvent system removes at least a portion of an asphaltene on the portion of the subterranean formation, and wherein the relative permeability modifier hinders subsequent asphaltene deposition on the portion of the subterranean formation.

In yet another embodiment, the present invention provides a method of removing asphaltenes and hindering subsequent asphaltene deposition in a subterranean formation comprising: introducing a permeability modifying fluid into a well bore that penetrates a subterranean formation, wherein the permeability modifying fluid comprises a relative permeability modifier; allowing at least a portion of the permeability modifying fluid to penetrate into a portion of the subterranean formation so that the relative permeability modifier present in the portion of the subterranean formation substantially diverts subsequently introduced aqueous fluids to less permeable portions of the subterranean formation, wherein the relative permeability modifier in the portion of the subterranean formation hinders subsequent asphaltene deposition; and introducing an asphaltene solvent system into the well bore to remove asphaltenes on the subterranean formation, wherein the relative permeability modifier present in the portion of the subterranean formation diverts the asphaltene solvent system to a less permeable portion of the subterranean formation.

The features and advantages of the present invention will be readily apparent to those skilled in the art. While numerous changes may be made by those skilled in the art, such changes are within the spirit of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to subterranean treatments and, more particularly, in one or more embodiments, to introducing a relative permeability modifier into a subterranean interval, optionally in conjunction with an asphaltene solvent system, to reduce asphaltene deposition.

Embodiments of the present invention relate to using a relative permeability modifier (e.g., water-soluble hydrophobically modified polymers), and optionally an asphaltene solvent system, to treat an interval of a subterranean formation. As used in this disclosure, the term “relative permeability modifier” refers to a polymer that selectively reduces the effective permeability of a subterranean formation to water-based fluids. In accordance with embodiments of the present invention, the relative permeability modifier may form a film on a well bore surface in the interval of the subterranean formation thereby decreasing water permeability and increasing the water film stability indefinitely. It is believed that the water film should hinder asphaltene deposition. Additionally, the relative permeability modifier may also be used to divert asphaltene solvent systems, for example, to less permeable portions of the formation. Otherwise, the asphaltene solvent system may preferentially enter portions of the interval with high permeability at the expense of portions of the interval with lesser permeability. Additionally, the relative permeability modifier and the asphaltene solvent system may be introduced simultaneously into the subterranean formation. As desired, embodiments of the present invention may use relative permeability modifiers, for example, to alleviate the necessity of using multiple asphaltene removal treatments.

I. Example Methods—Treatment of Formation Interval

A. Example Treatments with Relative Permeability Modifiers

The compositions disclosed herein may be used to treat an interval of a subterranean formation penetrated by a well bore. The interval may represent an interval that has been identified for treatment with a relative permeability modifier to hinder asphaltene deposition, in accordance with present embodiments. As will be appreciated by those of ordinary skill in the art, with the benefit of this disclosure, the interval may be any interval of a subterranean formation suitable for treatment. Moreover, as those of ordinary skill in the art will appreciate, with the benefit of this disclosure, embodiments of the present invention may be applicable for the treatment of both production and injection wells. Additionally, embodiments of the present invention also may be suitable for cased well bores or openhole well bores.

The interval identified for treatment may comprise an interval that has previously been treated with an asphaltene removal method. For example, the interval may have been treated with an asphaltene removal system involving heat, mechanical removal, a dispersant, or a solvent. Alternatively, the interval may have not been treated with an asphaltene removal system.

In accordance embodiments of the present invention, the interval may be contacted with a relative permeability modifier (e.g., a water-soluble hydrophobically modified polymer). In some embodiments, for contacting the interval with the relative permeability modifier, the relative permeability modifier may be present in a permeability modifying fluid introduced into the interval. Treatment fluids comprising the relative permeability modifier will be referred to herein as “permeability modifying fluids.” In some embodiments, the near well bore portion of the interval is contacted with the relative permeability modifier. Those of ordinary skill in the art will understand that the “near well bore portion” of a formation generally refers to the portion of a subterranean formation surrounding a well bore. For example, the “near well bore portion” may refer to the portion of the formation surrounding a well bore and having a depth of penetration of from about 1 to about 3 well bore diameters. In certain embodiments, the “near well bore portion” may refer to the portion of the formation surrounding a well bore in the range of about 30 feet to about 50 feet.

In general, the relative permeability modifier may attach to rock surfaces present in the interval and should increase the stability of the water film on the rock surfaces by increasing water retention. While not wishing to be limited to theory, it is believed that a considerable reduction in adsorbed asphaltenes is observed because the increased stability of the water film on rock surfaces helps reduce asphaltene adsorption rate and the presence of the relative permeability modifier reduces the surface availability of asphaltene particles to adsorb onto. Accordingly, it is believed that the asphaltene deposition rate onto the interval may greatly be reduced when production begins.

In certain embodiments, the contact of the interval with the relative permeability modifier should be controlled so that the flow of fluids (e.g., aqueous fluids) through the interval is not substantially prevented after the treatment with the relative permeability modifier. To prevent substantially reducing the flow of fluids through the interval after treatment, contact of the interval with the relative permeability modifier may be halted, if the injection pressure rises to 90% of the anticipated fracture gradient, and any subsequent fluid to be injected may be “spotted” to the interval before continuing injection into the interval. In certain embodiments the effective permeability of the interval to water should be at least about 1% to about 80% of its pre-treatment injectivity index (injection rate divided by injection pressure), alternatively, about 30% to about 40%. In certain embodiments, the interval may retain a water permeability sufficient to allow injection of water at a rate of about ¼ barrel per minute (about 10 gallons/minute). Examples of relative permeability modifiers suitable for use embodiments of the present invention are described in more detail below.

Any suitable technique may be used for introduction of the permeability modifying fluid into the interval, for example, bull heading, coil tubing, jointed pipe (e.g., with straddle packers, pinpoint injection tools, etc.) or any other suitable technique may be used. It should be noted that, to reduce the potential for the undesired fracturing of the interval, the permeability modifying fluid should be introduced into the interval at matrix flow rates. Example flow rates for the permeability modifying fluid are in the range of from about 0.25 barrels to about 3 barrels per minute. However, those of ordinary skill in the art will appreciate that these flow rates are merely examples, and embodiments of the present invention are applicable to flow rates outside these ranges. Further, as discussed previously, in certain embodiments, contacting the interval with the relative permeability modifier should be controlled so that the effective permeability of the interval is not undesirably reduced. For example, the pressure of the permeability modifying fluid may be monitored as it is being introduced into the interval. As the effective permeability to water of the interval decreases, due to the relative permeability modifier, there should be an increase in the pressure of the permeability modifying fluid. Therefore, this pressure may be monitored so that the permeability of the interval is not undesirably reduced to allow for the subsequent treatment of the interval. Other suitable techniques for monitoring the permeability of the interval also may be utilized.

B. Example Treatments with Relative Permeability Modifiers and Asphaltene Solvent Systems

The compositions disclosed herein may be used for the diversion of aqueous fluids in a variety of subterranean operations, such as in asphaltene removal operations. In some embodiments, the methods may comprise: introducing a permeability modifying fluid into a well bore that penetrates a subterranean formation, wherein the permeability modifying fluid comprises a relative permeability modifier; allowing at least a portion of the permeability modifying fluid to penetrate into a portion of the subterranean formation so that the relative permeability modifier present in the portion of the subterranean formation substantially diverts subsequently introduced aqueous fluids to less permeable portions of the subterranean formation, wherein the relative permeability modifier in the portion of the subterranean formation hinders subsequent asphaltene deposition; and introducing an asphaltene solvent system in to the well bore to remove asphaltenes on the subterranean formation, wherein the relative permeability modifier present in the portion of the subterranean formation diverts the asphaltene solvent system to a less permeable portion of the subterranean formation.

In certain embodiments, the methods may comprise: introducing a permeabiliy modifying fluid into a well bore that penetrates a subterranean formation, wherein the permeability modifying fluid comprises a relative permeability modifier and an asphaltene solvent system; allowing at least a portion of the permeability modifying fluid to penetrate into a portion of the subterranean formation so that the relative permeability modifier present in the portion of the subterranean formation substantially diverts subsequently introduced aqueous fluids to less permeable portions of the subterranean formation while increasing the water wettability of the formation.

It is believed that the relative permeability modifier attaches to surfaces within the porosity of the portion of the subterranean formation. Among other things, the presence of the relative permeability modifier in the portion of the subterranean formation should reduce the permeability thereof to aqueous fluids without substantially changing its permeability to hydrocarbons. Due to the reduction in the permeability of the portion of the subterranean formation, any aqueous fluid subsequently introduced into the well bore should be substantially diverted to another portion of the subterranean formation. Additionally, the relative permeability modifiers also may act to reduce subsequent problems associated with water flowing into the well bore from the subterranean formation. In some embodiments relative permeability modifier may be mixed with an aqueous fluid and introduced into a portion of the subterranean formation between stages of a treatment or as a pretreatment. In some embodiments, the asphaltene solvent systems of the present invention may be self-diverting. For example, in some embodiments, the relative permeability modifier may be included in the asphaltene solvent system during the subterranean treatment. In these embodiments, the relative permeability modifier may progressively divert the asphaltene solvent system to another portion of the subterranean formation. For instance, in some embodiments, as a first portion of the asphaltene solvent system penetrates into a portion of the subterranean formation a second portion of the asphaltene solvent system may be diverted to another portion of the subterranean formation.

In asphaltene removal operations, in some embodiments, a permeability modifying fluid of the present invention may be introduced into the subterranean formation between stages of the asphaltene removal operation, as a pretreatment, or as a combination thereof. For example, when the asphaltene removal operation is performed in stages, in the first stage an asphaltene solvent system may be introduced into a portion of the subterranean formation, followed by a permeability modifying fluid of the present invention. The relative permeability modifier present in the particular permeability modifying fluid of the present invention should reduce the permeability of the portion of the subterranean formation to aqueous fluids. The second stage of the asphaltene removal operation then may be substantially diverted to another portion of the subterranean formation. Alternating stages of the asphaltene solvent system and the permeability modifying fluid of the present invention may be continued as desired. In other embodiments, the permeability modifying fluids of the present invention may be used as a pretreatment. For instance, a permeability modifying fluid of the present invention may be introduced in a portion of the subterranean formation, wherein the relative permeability modifier present in the permeability modifying fluid of the present invention reduces the permeability of the portion of the subterranean formation to aqueous fluids. An asphaltene solvent system introduced into the well bore after the pretreatment, such as an asphaltene solvent system, may be substantially diverted to another portion of the subterranean formation.

II. Example Relative Permeability Modifiers

As described above, a relative permeability modifier may be introduced into at least a portion of a subterranean formation, in accordance with certain embodiments. In general, suitable relative permeability modifiers may be any of a variety of compounds that are capable of selectively reducing the effective permeability of a formation to water-based fluids without a comparable reduction of the formation's effective permeability to hydrocarbons. Suitable relative permeability modifiers generally include water-soluble polymers that attach to surfaces within the formation, reducing the water permeability without a comparable reduction in hydrocarbon permeability. As used in this disclosure, “water soluble” refers to at least about 0.0001 weight percent soluble in water. In certain embodiments, the water-soluble polymer is at least about 0.45 weight percent soluble in distilled water at room temperature. In certain embodiments, the water-soluble polymer is at least about 0.6 weight percent soluble in distilled water at room temperature.

Those of ordinary skill in the art, with the benefit of this disclosure, will appreciate that a variety of different water-soluble polymers may be suitable for use as the relative permeability modifiers. Examples of suitable water-soluble polymers include, but are not limited to, homo-, co-, and terpolymers of acrylamide, 2-acrylamido-2-methyl propane sulfonic acid, N,N-dimethylacrylamide, vinyl pyrrolidone, dimethylaminoethyl methacrylate, acrylic acid, dimethylaminopropylmethacrylamide, vinyl amine, vinyl acetate, trimethylammoniumethyl methacrylate chloride, methacrylamide, hydroxyethyl acrylate, vinyl sulfonic acid, vinyl phosphonic acid, methacrylic acid, vinyl caprolactam, N-vinylformamide, N,N-diallylacetamide, dimethyldiallyl ammonium halide, itaconic acid, styrene sulfonic acid, methacrylamidoethyltrimethyl ammonium halide, quaternary salt derivatives of acrylamide, quaternary salt derivatives of acrylic acid, and combinations thereof.

In addition, water-soluble polymers suitable for use as relative permeability modifiers also may include hydrophobically modified polymers. As used in this disclosure, the terms “hydrophobically modified,” “hydrophobic modification,” and the like refer to the incorporation into the hydrophilic polymer structure of hydrophobic groups, wherein the alkyl chain length is about 4 to about 22 carbons. While these hydrophobically modified polymers have hydrophobic groups incorporated into the hydrophilic polymer structure, they should remain water soluble. In some embodiments, a mole ratio of a hydrophilic monomer to the hydrophobic compound in the hydrophobically modified polymer is in the range of from about 99.98:0.02 to about 90:10, wherein the hydrophilic monomer is a calculated amount present in the hydrophilic polymer. In certain embodiments, the hydrophobically modified polymers may comprise a polymer backbone that comprises polar heteroatoms. Generally, the polar heteroatoms present within the polymer backbone of the hydrophobically modified polymers include, but are not limited to, oxygen, nitrogen, sulfur, or phosphorous.

Example hydrophobically modified polymers may contain a hydrophilic polymer backbone and a hydrophobic branch, wherein the hydrophobic branch includes an alkyl chain of about 4 to about 22 carbons. In certain embodiments, the hydrophobic branch may have an alkyl chain length of about 7 to about 22 carbons. In certain embodiments, the hydrophobic branch may have an alkyl chain length of about 12 to about 18 carbons.

Additional examples of suitable hydrophobically modified polymers include a polymer that has been hydrophobically modified with an alkyl group present on an amino group (in the polymer backbone or as a pendant group) in quaternized form. For example, an alkyl group may be present on a dialkyl amino pendant group in quaternized form. In one embodiment, the dialkyl amino pendant group comprises a dimethyl amino pendant group. One specific example of a hydrophobically modified polymer includes a polydimethylaminoethylmethacrylate or polydimethylaminopropylmethacrylamide that has been hydrophobically modified with an alkyl group with 4 carbons to 22 carbons (e.g., 4 carbons, 6, carbons, 8 carbons, 10 carbons, 12 carbons, 14 carbons, 16 carbons, 18 carbons, 20 carbons, 22 carbons, etc.) on a dimethylamino group. An example of a suitable hydrophobically modified polymer is HPT-1™ relative permeability modifying polymer available from Halliburton Energy Services, Inc., Duncan, Okla.

Examples of suitable hydrophobically modified polymers that may be utilized include, but are not limited to, acrylamide/octadecyldimethylammoniumethyl methacrylate bromide copolymer, dimethylaminoethyl methacrylate/vinyl pyrrolidone/hexadecyldimethylammoniumethyl methacrylate bromide terpolymer, and acrylamide/2-acrylamido-2-methyl propane sulfonic acid/2-ethylhexyl methacrylate terpolymer. Another example of a suitable hydrophobically modified polymer comprises an amino methacrylate/alkyl amino methacrylate copolymer. An example of a suitable amino methacrylate/alkyl amino methacrylate copolymer includes a dimethylaminoethyl methacrylate/alkyl-dimethylammoniumethyl methacrylate copolymer. An example of a suitable dimethylaminoethyl methacrylate/alkyl-dimethylammoniumethyl methacrylate copolymer includes a dimethylaminoethyl methacrylate/hexadecyl-dimethylammoniumethyl methacrylate copolymer. As discussed in more detail below, these copolymers may be formed, in embodiments, by reactions with a variety of alkyl halides. For example, in some embodiments, the hydrophobically modified polymer may comprise a dimethylaminoethyl methacrylate/hexadecyl-dimethylammoniumethyl methacrylate bromide copolymer.

Example hydrophobically modified polymers may be synthesized utilizing any suitable technique. For example, the hydrophobically modified polymers may be a reaction product of a reaction comprising a hydrophilic polymer and a hydrophobic compound. By way of further example, the hydrophobically modified polymers may be prepared from a polymerization reaction comprising a hydrophilic monomer and a hydrophobically modified hydrophilic monomer. In certain embodiments, the hydrophobically modified polymers may be pre-reacted before they are placed into the well bore. Alternatively, in some embodiments, the hydrophobically modified polymers may be prepared by an appropriate in situ reaction. Suitable hydrophobically modified polymers and methods for their preparation are described in more detail in U.S. Pat. Nos. 6,476,169 and 7,117,942, the disclosures of which are incorporated herein by reference. Those of ordinary skill in the art, with the benefit of this disclosure, will be able to determine other suitable methods for the synthesis of suitable hydrophobically modified polymers.

In certain embodiments, suitable hydrophobically modified polymers may be synthesized by the hydrophobic modification of a hydrophilic polymer via reaction with a hydrophobic compound. As described above, hydrophobic modification refers to incorporation into the hydrophilic polymer structure of hydrophobic groups, wherein the alkyl chain length is from about 4 to about 22 carbons. The hydrophilic polymers suitable for forming the hydrophobically modified polymers used in the present invention should be capable of reacting with hydrophobic compounds. Suitable hydrophilic polymers include, homo-, co-, or terpolymers such as, but not limited to, polyacrylamides, polyvinylamines, poly(vinylamines/vinyl alcohols), alkyl acrylate polymers in general, and combinations thereof. Additional examples of alkyl acrylate polymers include polydimethylaminoethyl methacrylate, polydimethylaminopropyl methacrylamide, poly(acrylamide/dimethylaminoethyl methacrylate), poly(methacrylic acid/dimethylaminoethyl methacrylate), poly(2-acrylamido-2-methyl propane sulfonic acid/dimethylamino ethyl methacrylate), poly(acrylamide/dimethylaminopropyl methacrylamide), poly (acrylic acid/dimethylaminopropyl methacrylamide), poly(methacrylic acid/dimethylaminopropyl methacrylamide), and combinations thereof. In certain embodiments, the hydrophilic polymers comprise a polymer backbone and reactive amino groups in the polymer backbone or as pendant groups, the reactive amino groups capable of reacting with hydrophobic compounds. In some embodiments, the hydrophilic polymers comprise dialkyl amino pendant groups. In some embodiments, the hydrophilic polymers comprise a dimethyl amino pendant group and a monomer comprising dimethylaminoethyl methacrylate or dimethylaminopropyl methacrylamide. In certain embodiments, the hydrophilic polymers comprise a polymer backbone that comprises polar heteroatoms, wherein the polar heteroatoms present within the polymer backbone of the hydrophilic polymers include oxygen, nitrogen, sulfur, or phosphorous. Suitable hydrophilic polymers that comprise polar heteroatoms within the polymer backbone include homo-, co-, or terpolymers, such as, but not limited to, celluloses, chitosans, polyamides, polyetheramines, polyethyleneimines, polyhydroxyetheramines, polylysines, polysulfones, gums, starches, and combinations thereof. In one embodiment, the starch is a cationic starch. A suitable cationic starch may be formed by reacting a starch, such as corn, maize, waxy maize, potato, tapioca, or the like, with the reaction product of epichlorohydrin and trialkylamine.

The hydrophobic compounds that are capable of reacting with the hydrophilic polymers include alkyl halides, sulfonates, sulfates, organic acids, and organic acid derivatives. Examples of suitable organic acids and derivatives thereof include, but are not limited to, octenyl succinic acid; dodecenyl succinic acid; and anhydrides, esters, imides, and amides of octenyl succinic acid or dodecenyl succinic acid. In certain embodiments, the hydrophobic compounds may have an alkyl chain length of from about 4 to about 22 carbons. In another embodiment, the hydrophobic compounds may have an alkyl chain length of from about 7 to about 22 carbons. In another embodiment, the hydrophobic compounds may have an alkyl chain length of from about 12 to about 18 carbons. For example, where the hydrophobic compound is an alkyl halide, the reaction between the hydrophobic compound and hydrophilic polymer may result in the quaternization of at least some of the hydrophilic polymer amino groups with an alkyl halide, wherein the alkyl chain length is from about 4 to about 22 carbons.

As previously mentioned, in certain embodiments, suitable hydrophobically modified polymers also may be prepared from a polymerization reaction comprising a hydrophilic monomer and a hydrophobically modified hydrophilic monomer. The hydrophobically modified polymers synthesized from the polymerization reactions may have estimated molecular weights in the range of from about 100,000 to about 10,000,000 and mole ratios of the hydrophilic monomer(s) to the hydrophobically modified hydrophilic monomer(s) in the range of from about 99.98:0.02 to about 90:10.

A variety of hydrophilic monomers may be used to form the hydrophobically modified polymers useful in the present invention. Examples of suitable hydrophilic monomers include, but are not limited to, acrylamide, 2-acrylamido-2-methyl propane sulfonic acid, N,N-dimethylacrylamide, vinyl pyrrolidone, dimethylaminoethyl methacrylate, acrylic acid, dimethylaminopropylmethacrylamide, vinyl amine, vinyl acetate, trimethylammoniumethyl methacrylate chloride, methacrylamide, hydroxyethyl acrylate, vinyl sulfonic acid, vinyl phosphonic acid, methacrylic acid, vinyl caprolactam, N-vinylformamide, N,N-diallylacetamide, dimethyldiallyl ammonium halide, itaconic acid, styrene sulfonic acid, methacrylamidoethyltrimethyl ammonium halide, quaternary salt derivatives of acrylamide, quaternary salt derivatives of acrylic acid, and combinations thereof.

A variety of hydrophobically modified hydrophilic monomers also may be used to form the hydrophobically modified polymers useful in certain embodiments. Examples of suitable hydrophobically modified hydrophilic monomers include, but are not limited to, alkyl acrylates, alkyl methacrylates, alkyl acrylamides, alkyl methacrylamides alkyl dimethylammoniumethyl methacrylate halides, and alkyl dimethylammoniumpropyl methacrylamide halides, wherein the alkyl groups have from about 4 to about 22 carbon atoms. In another embodiment, the alkyl groups have from about 7 to about 22 carbons. In another embodiment, the alkyl groups have from about 12 to about 18 carbons. In certain embodiments, the hydrophobically modified hydrophilic monomer comprises octadecyldimethylammoniumethyl methacrylate bromide, hexadecyldimethylammoniumethyl methacrylate bromide, hexadecyldimethylammoniumpropyl methacrylamide bromide, 2-ethylhexyl methacrylate, or hexadecyl methacrylamide.

In addition, water-soluble polymers suitable for use as relative permeability modifiers also may include hydrophilically modified polymers. As used in this disclosure, the terms “hydrophilic modification,” “hydrophilically modified,” and the like refer to the incorporation into the hydrophilic polymer structure of hydrophilic groups, such as to introduce branching or to increase the degree of branching in the hydrophilic polymer. The hydrophilically modified polymers of certain embodiments typically have molecular weights in the range of from about 100,000 to about 10,000,000. In certain embodiments, the hydrophilically modified polymers comprise a polymer backbone, the polymer backbone comprising polar heteroatoms. Generally, the polar heteroatoms present within the polymer backbone of the hydrophilically modified polymers include, but are not limited to, oxygen, nitrogen, sulfur, or phosphorous.

The hydrophilically modified polymers may be synthesized using any suitable method. In one example, the hydrophilically modified polymers may be a reaction product of a hydrophilic polymer and a hydrophilic compound. In certain embodiments, suitable hydrophilically modified polymers may be formed by additional hydrophilic modification, for example, to introduce branching or to increase the degree of branching, of a hydrophilic polymer. Those of ordinary skill in the art, with the benefit of this disclosure, will be able to determine other suitable methods for the preparation of suitable hydrophilically modified polymers.

The hydrophilic polymers suitable for forming the hydrophilically modified polymers used in certain embodiments should be capable of reacting with hydrophilic compounds. In certain embodiments, suitable hydrophilic polymers include, homo-, co-, or terpolymers, such as, but not limited to, polyacrylamides, polyvinylamines, poly(vinylamines/vinyl alcohols), and alkyl acrylate polymers in general. Additional examples of alkyl acrylate polymers include, but are not limited to, polydimethylaminoethyl methacrylate, polydimethylaminopropyl methacrylamide, poly(acrylamide/dimethylaminoethyl methacrylate), poly(methacrylic acid/dimethylaminoethyl methacrylate), poly(2-acrylamido-2-methyl propane sulfonic acid/dimethyl amino ethyl methacrylate), poly(acrylamide/dimethylaminopropyl methacrylamide), poly(acrylic acid/dimethylaminopropyl methacrylamide), and poly(methacrylic acid/dimethylaminopropyl methacrylamide). In certain embodiments, the hydrophilic polymers comprise a polymer backbone and reactive amino groups in the polymer backbone or as pendant groups, the reactive amino groups capable of reacting with hydrophilic compounds. In some embodiments, the hydrophilic polymers comprise dialkyl amino pendant groups. In some embodiments, the hydrophilic polymers comprise a dimethyl amino pendant group and at least one monomer comprising dimethylaminoethyl methacrylate or dimethylaminopropyl methacrylamide. In other embodiments, the hydrophilic polymers comprise a polymer backbone comprising polar heteroatoms, wherein the polar heteroatoms present within the polymer backbone of the hydrophilic polymers include, but are not limited to, oxygen, nitrogen, sulfur, or phosphorous. Suitable hydrophilic polymers that comprise polar heteroatoms within the polymer backbone include homo-, co-, or terpolymers, such as, but not limited to, celluloses, chitosans, polyamides, polyetheramines, polyethyleneimines, polyhydroxyetheramines, polylysines, polysulfones, gums, starches, and derivatives thereof. In one embodiment, the starch is a cationic starch. A suitable cationic starch may be formed by reacting a starch, such as corn, maize, waxy maize, potato, tapioca, and the like, with the reaction product of epichlorohydrin and trialkylamine.

Hydrophilic compounds suitable for reaction with the hydrophilic polymers include, but are not limited to: polyethers that comprise halogens; sulfonates; sulfates; organic acids; and organic acid derivatives. Examples of suitable polyethers include, but are not limited to, polyethylene oxides, polypropylene oxides, and polybutylene oxides, and copolymers, terpolymers, and mixtures thereof. In some embodiments, the polyether comprises an epichlorohydrin-terminated polyethylene oxide methyl ether.

The hydrophilically modified polymers formed from the reaction of a hydrophilic polymer with a hydrophilic compound may have estimated molecular weights in the range of from about 100,000 to about 10,000,000 and may have weight ratios of the hydrophilic polymers to the polyethers in the range of from about 1:1 to about 10:1. Examples of suitable hydrophilically modified polymers having molecular weights and weight ratios in the ranges set forth above include, but are not limited to, the reaction product of polydimethylaminoethyl methacrylate and epichlorohydrin-terminated polyethyleneoxide methyl ether; the reaction product of polydimethylaminopropyl methacrylamide and epichlorohydrin-terminated polyethyleneoxide methyl ether; and the reaction product of poly(acrylamide/dimethylaminopropyl methacrylamide) and epichlorohydrin-terminated polyethyleneoxide methyl ether. In some embodiments, the hydrophilically modified polymer comprises the reaction product of a polydimethylaminoethyl methacrylate and epichlorohydrin-terminated polyethyleneoxide methyl ether having a weight ratio of polydimethylaminoethyl methacrylate to epichlorohydrin-terminated polyethyleneoxide methyl ether of about 3:1.

III. Example Permeability Modifying Fluids

In accordance with some embodiments, the relative permeability modifier may be present in a permeability modifying fluid introduced into the subterranean formation. Sufficient concentrations of the relative permeability modifier should be present in these permeability modifying fluids to provide the desired level of permeability modification. In some embodiments, the relative permeability modifier may be present in these permeability modifying fluids in an amount in the range of from about 0.02% to about 10% by weight of the permeability modifying fluid. In another embodiment, relative permeability modifier may be present in these permeability modifying fluids in an amount in the range of from about 0.05% to about 1.0% by weight of the permeability modifying fluid. In certain embodiments, the relative permeability modifier may be provided in a concentrated aqueous solution prior to its combination with the other components necessary to form the permeability modifying fluids.

In addition to the relative permeability modifier, the permeability modifying fluids generally also comprise water. The water included in the permeability modifying fluid may include freshwater, saltwater (e.g., water containing one or more salts dissolved therein), brines (e.g., natural or produced brines), seawater, or another other aqueous fluid that does not undesirably effect the other components in the permeability modifying fluid.

IV. Example Asphaltene Solvent Systems

As described above, an asphaltene solvent system may be introduced into at least a portion of a subterranean formation, in accordance with certain embodiments. In general, suitable asphaltene solvent systems may include any solvent system able to remove asphaltene from a subterranean formation. In some embodiments, the asphaltene solvent system may comprise water, an organic solvent, and a surfactant. In certain embodiments, the asphaltenes solvent system may be a weak emulsion or dispersion.

The water included in the asphaltene solvent system may include freshwater, saltwater (e.g., water containing one or more salts dissolved therein), brines (e.g., natural or produced brines), seawater, or another other aqueous fluid that does not undesirably effect the other components in the permeability modifying fluid. In some embodiments the water may comprise an acid. Preferably, the water may comprise a water-soluble salt. In certain embodiments, the water is present in asphaltene solvent system in an amount in the range of about 50% to about 100% by volume of the asphaltene solvent system. In certain embodiments, the water is present in the asphaltene solvent system in the range of about 65% to about 85% by volume of the asphaltene solvent system. In certain embodiments, the water is present in the asphaltene solvent system in the range of about 70% to about 80% by volume of the asphaltene solvent system. One of ordinary skill in the art with the benefit of this disclosure will recognize the appropriate amount of water for a chosen application. Furthermore, in certain embodiments, the water may comprise a permeability modifying fluid.

The organic solvent utilized in embodiments of the asphaltene solvent systems of the present invention may comprise any suitable non-polar organic solvent or polar organic solvent. In certain embodiments, the organic solvent may be present in the asphaltene solvent system in an amount in the range of about 30% to about 90% by volume of the asphaltene solvent system. In certain embodiments, the organic solvent may be present in the asphaltene solvent system in an amount in the range of about 30% to about 70% by volume of the asphaltene solvent system. In certain embodiments, the organic solvent may be present in the asphaltene solvent system in an amount in the range of about 40% to about 60% by volume of the asphaltene solvent system.

Suitable examples of non-polar organic solvents include aromatic solvents, terpenes, kerosene, diesel, and any combination thereof. Examples of suitable aromatic solvents include heavy aromatics, light aromatics, xylene, toluene and naptha. Other suitable non-polar organic solvents may include D-limonene, dipentene, and terpenes. Suitable examples of polar organic solvents include N-methyl pyrrolidone and cyclohexanone.

The solvents should be selected for being effective to substantially dissolve asphaltenes. Other consideration in selecting a solvent is that the components should not be incompatible with the formation fluids to avoid the formation of undesirable precipitates or residues. Other considerations include that the solvent should not tend to poison any catalysis used in refining the hydrocarbon produced form the well.

In some embodiments, the organic solvent may comprise an organic solvent blend. In some embodiments, the organic solvent blend may comprise a non-polar organic solvent and a polar organic solvent. In certain embodiments, the organic solvent blend comprise the non-polar organic solvent in an amount of about 90% to about 99.9% by volume of the organic solvent blend, and the polar organic solvent in an amount of about 0.1% to about 10% by volume of the organic solvent blend. In certain embodiments, the organic solvent blend comprises the non-polar organic solvent in an amount of about 95% to about 99% by volume of the organic solvent blend and the polar organic solvent in an amount of about 1% to about 5% by volume of the organic solvent blend.

The specific organic solvent blend may be selected for being effective to substantially dissolve asphaltenes. As well known in the art, the exact composition and nature of asphaltenes can vary widely depending on the source, and it can be desirable to adjust or modify the exact solvent blend and the water-solvent emulsion compositions depending on the source of the asphaltenes. For example, a composition according to the invention can be more particularly adapted for asphaltenes of the types found in Italy or Northern Africa

Another consideration in selecting the organic solvent blend is that the components should not be incompatible with the formation fluids to avoid the formation of undesirable precipitates or residues. Other considerations include that the solvent blend should not tend to poison any catalysts used in the refining of the hydrocarbon produced from the well.

The flash point of the organic solvent blend may yet be another consideration. For example, the flash point of each of the organic solvents, whether non-polar or polar, in the organic solvent blend may be greater than 40° C., and, alternatively, may be greater than 50° C. The flash point of xylene, for example, is only 27° C. The non-polar organic solvent in the organic solvent blend can comprise, for example, a mixture of D-limonene and dipentene, for which some mixtures have a flash point of about 47° C. An example of a suitable non-polar solvent for use in the solvent blend is a terpene blend that has a flash point of greater than 50° C. In certain embodiments, a “heavy aromatic solvent” may be used, which is a distillation cut of a crude oil from which light aromatic solvents, such as xylene and toluene, have been previously distilled out.

In accordance with one embodiment of the present invention, the polar organic solvent may comprise at least two different polar organic solvents. The polar organic solvent may be selected for its ability to enhance the solubility of asphaltenes in the organic solvent blend relative to the solubility of the asphaltenes in the non-polar organic solvent alone. A suitable polar organic solvent may be selected from the group consisting of N-methyl pyrrolidone, which has a high flash point of 92° C., and cyclohexanone, which has an adequately high flash point of 44° C., and any combination thereof in any proportion. Without being limited by theory, it is believed that the combination of two different polar organic solvents helps dissolve the asphaltenes better than the use of either of these two solvents alone. Toluene has a reported Snyder polarity index of only about 2.3, and toluene is normally considered to be a non-polar organic solvent. Cyclohexanone has a reported Snyder polarity index of 4.5, and N-methyl pyrrolidone has a reported Snyder polarity index of about 6.5. These polarity indices provide two different intermediate steps in polarity between non-polar solvents, such as toluene, and water, which has a Snyder polarity index of 9. It is believed that using at least two polar organic solvents having substantially different polarities is contributing to the unexpectedly improved results in dissolving asphaltenes. Accordingly, it is believed that other combinations of polar organic solvents will be suitable, especially if the polar organic solvents have substantially different polarities. Accordingly, in certain embodiments, two different polar organic solvents may be used with each solvent having a Snyder polarity index between about 3 and about 7. By way of example, one of the polar organic solvents may have a Snyder polarity index in the range of about 3 to about 5 and one of the polar organic solvents may have a Snyder polarity index in the range of about 5 to about 7. In certain embodiments, at least two of the polar organic solvents may have a Snyder polarity indexes that are at least about 1.5 polarity index units apart.

The surfactant included in the asphaltene solvent systems may be any surfactant that is capable of forming a weak emulsion (preferably a water-external emulsion) or a dispersion of the water and the solvent. For example, the surfactant may comprise a water-soluble surfactant. The surfactant may be present in the asphaltene solvent system in an amount of about 0.1% to about 10% by volume of the asphaltene solvent system. In certain embodiments, the surfactant may be present in the asphaltene solvent system in an amount of about 0.3% to about 6% by volume of the asphaltene solvent system. In certain embodiments, the surfactant may be present in the asphaltene solvent system in an amount of about 0.3% to about 0.4% by volume of the asphaltene solvent system. Suitable surfactants may include ethoxylated alcohols, ethoxylated nonyphenol, and any combination thereof.

In certain embodiments, the flash point of the surfactant may be greater than about 40° C., and, alternatively, greater than about 50° C. “Baraklean” is a suitable example of a blend of water-soluble surfactants and has a flash point above about 93° C. (about 200° F.), which is commercially available from Baroid Fluid Services. “Baraklean NS” or “Baraklean NS plus” are also suitable, being a blend of water-soluble surfactants with a complexing agent.

To facilitate a better understanding of the present invention, the following examples of certain aspects of some embodiments are given. In no way should the following examples be read to limit, or define, the entire scope of the invention.

EXAMPLE 1

The solubility of asphaltene in various crude samples obtained from Australia was measured. A portion of each test sample (Sample No. 1, Sample No. 2, and Sample No. 3) was treated with either recipe 1 or recipe 2 in six separate tests. Recipe 1 comprised 35% vol/vol Paragon 100E+(heavy aromatic naptha, 5% vol/vol Targon II methyl pyrrolidone, 5% vol/vol Paragon 1(terpenes), 0.4% vol/vol WS-36M, and 54.6% vol/vol Fe-Acid. Recipe 2 comprised 35% vol/vol Paragon 100E+, 5% vol/vol Targon II, 5% Paragon 1, 0.4% vol/vol WS-36M, and 54.6% vol/vol of a mixture comprising 93.3% vol/vol FE-Acid and 6.7% vol/vol of a relative permeability modifier. A known amount of asphaltene was added to a testing vial. The solvent blend was then added. After a predetermined amount of time and temperature. The undissolved asphaltene was determined using a gravimetric testing method. The results of the tests are also set forth in Table I below.

TABLE 1 Initial Weight Percent Weight of Final Weight Percent Test Sample Recipe of Paraffin Asphaltene of Paraffin Dissolved No. No. No. (g) (%) (g) (%) 1 1 1 2.000 20.000 1.536 23.208 2 2 1 2.003 20.000 1.973 1.473 3 3 1 2.002 20.000 1.349 32.621 4 1 2 1.432 14.32 0.973 32.053 5 2 2 2.000 20.000 1.504 24.800 6 3 2 2.000 20.000 1.319 34.050

As can be seen by table 1, the use of a relative permeability modifier and solvent blend may enhance the dissolving nature of an asphaltene solvent system.

Therefore, the present invention is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the present invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the present invention. All numbers and ranges disclosed above may vary by some amount. Whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any included range falling within the range is specifically disclosed. In particular, every range of values (of the form, “from about a to about b,” or, equivalently, “from approximately a to b,” or, equivalently, “from approximately a-b”) disclosed herein is to be understood to set forth every number and range encompassed within the broader range of values. Moreover, the indefinite articles “a” or “an”, as used in the claims, are defined herein to mean one or more than one of the element that it introduces. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. 

1. A method of hindering asphaltene deposition comprising: identifying an interval of the subterranean formation to be treated with a relative permeability modifier to hinder subsequent asphaltene deposition; introducing the relative permeability modifier into the subterranean formation; and allowing the relative permeability modifier to contact the interval, thereby attaching to surfaces within the subterranean formation and hindering subsequent asphaltene deposition.
 2. The method of claim 1 wherein the relative permeability modifier comprises a water-soluble hydrophobically modified polymer.
 3. The method of claim 2 wherein the hydrophobically modified polymer comprises a hydrophilic polymer backbone and a hydrophobic branch, the hydrophobic branch comprising about 7 to about 22 carbons.
 4. The method of claim 2 wherein the hydrophobically modified polymer comprises an alkyl group present on an amino group in quaternized form.
 5. The method of claim 2 wherein the hydrophobically modified polymer comprises polydimethylaminoethylmethacrylate or polydimetylaminopropylmethacrylamide that has been hydrophobically modified with an alkyl group of about 12 to about 22 carbons.
 6. A method of removing asphaltenes and hindering subsequent asphaltene deposition in a subterranean formation comprising: introducing a fluid comprising a relative permeability modifier and an asphaltene solvent system into the subterranean formation; and allowing the fluid to contact a portion of the subterranean formation, wherein the asphaltene solvent system removes at least a portion of an asphaltene on the portion of the subterranean formation, and wherein the relative permeability modifier hinders subsequent asphaltene deposition on the portion of the subterranean formation.
 7. The method of claim 6 comprising identifying an interval of the subterranean formation to be treated with the relative permeability modifier to hinder subsequent asphaltene deposition
 8. The method of claim 6 wherein the relative permeability modifier comprises a water-soluble hydrophobically modified polymer.
 9. The method of claim 8 wherein the hydrophobically modified polymer comprises a hydrophilic polymer backbone and a hydrophobic branch, the hydrophobic branch comprising about 7 to about 22 carbons.
 10. The method of claim 8 wherein the hydrophobically modified polymer comprises an alkyl group present on an amino group in quaternized form.
 11. The method of claim 8 wherein the hydrophobically modified polymer comprises polydimethylaminoethylmethacrylate or polydimetylaminopropylmethacrylamide that has been hydrophobically modified with an alkyl group of about 12 to about 22 carbons.
 12. The method of claim 6 wherein asphaltene solvent system comprises water, an organic solvent, a surfactant.
 13. The method of claim 12 wherein the organic solvent comprises at least one non-polar organic solvent selected from the group consisting of an aromatic solvent, a terpene, kerosene, diesel, xylene, toluene, cyclohexanone, D-limonene, and dipentene
 14. The method of claim 12 wherein the organic solvent comprises at least one polar organic solvent selected from the group consisting of N-methyl pyrrolidone and cyclohexanone.
 15. The method of claim 6 wherein the asphaltene solvent system comprises at least two polar organic solvents, wherein one of the polar organic solvents has a Snyder polarity index in the range of about 3 to about 5, and wherein another of the polar organic solvents has a Snyder polarity index in the range of about 5 to about
 7. 16. A method of removing asphaltenes and hindering subsequent asphaltene deposition in a subterranean formation comprising: introducing a permeability modifying fluid into a well bore that penetrates a subterranean formation, wherein the permeability modifying fluid comprises a relative permeability modifier; allowing at least a portion of the permeability modifying fluid to penetrate into a portion of the subterranean formation so that the relative permeability modifier present in the portion of the subterranean formation substantially diverts subsequently introduced aqueous fluids to less permeable portions of the subterranean formation, wherein the relative permeability modifier in the portion of the subterranean formation hinders subsequent asphaltene deposition; and introducing an asphaltene solvent system into the well bore to remove asphaltenes on the subterranean formation, wherein the relative permeability modifier present in the portion of the subterranean formation diverts the asphaltene solvent system to a less permeable portion of the subterranean formation.
 17. The method of claim 16 comprising identifying an interval of the subterranean formation to be treated with the permeability modifying fluid to hinder subsequent asphaltene deposition
 18. The method of claim 16 wherein the relative permeability modifier comprises a water-soluble hydrophobically modified polymer.
 19. The method of claim 18 wherein the hydrophobically modified polymer comprises a hydrophilic polymer backbone and a hydrophobic branch, the hydrophobic branch comprising about 7 to about 22 carbons.
 20. The method of claim 18 wherein the hydrophobically modified polymer comprises an alkyl group present on an amino group in quaternized form.
 21. The method of claim 18 wherein the hydrophobically modified polymer comprises polydimethylaminoethylmethacrylate or polydimetylaminopropylmethacrylamide that has been hydrophobically modified with an alkyl group of about 12 to about 22 carbons.
 22. The method of claim 16 wherein asphaltene solvent system comprises water, an organic solvent, a surfactant.
 23. The method of claim 22 wherein the organic solvent comprises at least one non-polar organic solvent selected from the group consisting of an aromatic solvent, a terpene, kerosene, diesel, xylene, toluene, cyclohexanone, D-limonene, and dipentene
 24. The method of claim 22 wherein the organic solvent comprises at least one polar organic solvent selected from the group consisting of N-methyl pyrrolidone and cyclohexanone.
 25. The method of claim 16 wherein the asphaltene solvent system comprises at least two polar organic solvents, wherein one of the polar organic solvents has a Snyder polarity index in the range of about 3 to about 5, and wherein another of the polar organic solvents has a Snyder polarity index in the range of about 5 to about
 7. 